1. Field of the Invention
The invention relates generally to the field of optical sensing systems. More specifically, the invention relates to optical sensing systems using various multiplexing techniques to operate a plurality of individual seismic sensors on a sensor cable.
2. Background Art
Optical systems for sensing physical properties such as acceleration, motion and/or pressure are used for, among other purposes, sensing seismic energy from the Earth's subsurface. The seismic energy may be naturally occurring, or may be imparted by a seismic energy source for the purpose of performing reflection seismic surveys. Detecting seismic energy may include detecting pressure, or changes in pressure, in a body of water. A sensor used to measure such changes in pressure is known as a hydrophone. Detecting seismic energy also includes detecting motion on or near the Earth's surface, or in a body of water. Motion may be detected using devices known as geophones. Geophone signals are related to velocity of motion. Accelerometers, which produce signals related to the time derivative of velocity of motion (acceleration), are also used to detect seismic energy. Sensors known in the art which respond to the foregoing physical parameters generate an optical signal in response to the detected physical parameter. The optical signal may be, for example, a change in wavelength, a change in phase or an interference pattern in response to changes in the physical parameter. Means for distributing light to and collecting the light from a plurality of the optical sensors is referred to as optical telemetry. Many individual optical sensors can be multiplexed from relatively few light source and signal return optical fibers using optical telemetry systems known in the art.
Generally, optical telemetry known in the art includes time division, frequency division and/or wavelength division multiplexing (TDM, FDM and/or WDM, respectively). A selected length of optical fiber ultimately affixed to an optical sensing device carries light from a source, which is distributed to the various optical sensors in a sensing system. The light in the sensor experiences a change or phase shift related to the physical property being measured. The change in optical characteristics of the optical fiber that causes changes in the properties of the applied light may be detected by one of a number of different optical measurement techniques. Optical signals from the sensors are then collected and returned to a receiving device for demultiplexing and analyzing the signals from each optical sensor.
A fiber optic telemetry disclosed, for example, in U.S. Pat. No. 4,648,083 issued to Gialorenzi is a typical fiber optic system using both FDM and WDM telemetry. The fiber optic telemetry disclosed in the '083 patent includes optical fiber that is distributed to and from an optoelectronic cabinet. Individual sensor “channels” are multiplexed in an M×N fiber distribution and collection FDM and/or WDM arrangement. A number “M” of input fibers and a number “N” of signal return fibers are used to operate a number M×N number of individual optical channels. U.S. Pat. No. 5,696,857 issued to Frederick discloses a WDM/FDM scheme using WDM tap couplers to drop an individual wavelength to a group of optical sensors. U.S. Pat. No. 5,866,898 issued to Hodgson discloses a scheme that use distribution and return buses with optical amplifiers to maintain suitable power levels in the returned optical signals.
U.S. Pat. No. 6,850,461 issued to Maas et al. and assigned to the assignee of the present invention discloses a seismic cable system using WDM and/or FDM techniques in which optical splitting of source light from an input bus to individual sensors and recombination of signals from the individual sensors are made in discrete modules, such that optical splicing and splitting or recombining components are mechanically isolated from other portions of the cable. Such arrangement is intended to improve the reliability of seismic sensing systems by isolating failure prone system elements to easily replaceable modules.
Seismic cables are subjected to rough handling and severe environmental conditions, sometimes resulting in breakage of one or more optical fibers in the cable, notwithstanding well designed construction. In some cases, breakage of an input bus fiber or a return bus fiber in a sensor cable such as the ones described in the above referenced patents may result in loss of signals from a significant portion, or even all of the sensors in an individual seismic sensor cable.
What is needed is a seismic sensing cable that provides redundant light source and signal return bus capability to increase survivability of the cable in the event of bus fiber failure.